Method of electrophotography

ABSTRACT

In a method of electrophotography wherein use is made of a photosensitive element including a highly insulative layer, a photoconductive layer manifesting persistent internal polarization, and an electrode layer transparent to visible light rays which are bonded together into an integral structure, and a latent image is formed on the surface of the highly insulative layer by depositing a charge of one polarity on the surface of the highly insulative layer and then depositing a charge of the opposite polarity concurrently with the projection of a light image through the transparent electrode, the highly insulative layer includes a mixture of a highly insulative material and a material highly reflective to visible light rays and the latent image is developed with a colored toner under ambient light while correcting the powder image by adjusting the density of the toner.

United States Patent [191 Kinoshita Dec. 23, 1975 [73] Assignee:Katsuragawa Denki Kabushiki Kaisha, Tokyo, Japan [22] Filed: Aug. 15,1973 [21] Appl. No.: 388,638

Related US. Application Data [63] Continuation-in-part of Ser. No.169,654, Aug. 6,

1971, abandoned.

[52] US. Cl 96/1 R; 96/67; 96/82 [51] Int. Cl G03g 13/22 [58] Field ofSearch 96/1 R, 1.5, 82

[56] References Cited UNITED STATES PATENTS 2,773,769 12/1956 Goldschein96/87 2,886,434 5/1959 Owens 96/1.5

2,959,153 11/1960 Hider 96/1 R 3,265,595 8/1966 Urbach 96/1 R 3,511,6515/1970 Rosenberg 96/l.5 3,642,470 2/1972 Tavemier et al. 96/1.53,653,064 3/1972 Inoue et a1. 96/1 R FOREIGN PATENTS OR APPLICATIONS812,419 4/1959 United Kingdom 96/1 R Primary Examiner-J. Travis BrownAttorney, Agent, or FirmBosworth, Sessions & McCoy [57] ABSTRACT In amethod of electrophotography wherein use is made of a photosensitiveelement including a highly insulative layer, a photoconductive layermanifesting persistent internal polarization, and an electrode layertransparent to visible light rays which are bonded together into anintegral structure, and a latent image is formed on the surface of thehighly insulative layer by depositing a charge of one polarity on thesurface of the highly insulative layer and then depositing a charge ofthe opposite polarity concurrently with the projection of a light imagethrough the transparent electrode, the highly insulative layer includesa mixture of a highly insulative material and a material highlyreflective to visible light rays and the latent image is developed witha colored toner under ambient light while correcting the powder image byadjusting the density of the toner.

16 Claims, 3 Drawing Figures US. Patent Dec. 23, 1975 Sheet 1 of23,928,031

FIG.

INVENTOR KO/cH/ /(//\/05H W BY flaw/1 M",

M Y- QMLJV ATTORNEY US. Patent Dec.23,1975 Sheet2of2 3,928,031

FIG.3

WElGHT OF TiO2 METHOD OF ELECTROPIIOTOGRAPHY CROSS REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of applicationSer. No. 169,654 filed Aug. 6, 1971 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an improvedphotosensitive element and a method of electrophotography utilizing thesame in which the developed powder image can be observed while it isformed.

I have already invented a method of electrophotography as disclosed inUS. Pat. Nos. 3,457,070 dated July 22, I969 and 3,536,483 dated Oct. 27,1970. According to this method an electrostatic latent image is formedby the steps of applying a first electric field across a photosensitiveelement including a transparent highly insulative layer, aphotoconductive layer exhibiting persistent internal polarization and anelectrode layer which are laminated in the order mentioned and bondedtogether into an integral structure whereby to deposit a charge of onepolarity on the surface of the highly insulative layer; applying asecond electric field across the photosensitive element to deposit acharge of the opposite polarity on the surface of the highly insulativelayer; and projecting a light image on the photoconductive layerconcurrently with the application of the second electric field wherebyto form a latent image corresponding to the light image on the surfaceof the highly insulative layer. A second highly insulative layer may beinterposed between the photoconductive layer and the electrode layer toimprove the signal-to-noise ratio. Usually, the light image is projectedupon the photoconductive layer through the highly insulative layer sothat the highly insulative layer should be transparent to visible lightrays.

The electrostatic latent image thus formed is then developed into avisible powder image under ambient light by ineans of colored(ordinarily black) toner, and the powder image is transfer printed ontoa recording medium to produce a permanent copy. Due to large absorptionof incident light rays by the photoconduc: tive layer, the color thereofbecomes black or deep red so that where the latent image is developedwith black toner, it is impossible to observe or supervise the progressof development or the variation of color during the course ofdevelopment because the light reflected by the photoconductive layerprevents such observation. For this reason, even though the method ofelectrophotography disclosed in the foregoing patents permitsdevelopment and transfer printing under ambient light or uniform light,it is impossible to observe, correct or modify the progress ofdevelopment to obtain valuable information or to use the developedpowder image as a signal source.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved method of electrophotography or radiation electrophotography inwhich the quality of the powder image can be readily controlled byadjusting the density of the toner while observing the powder image asit is being formed.

According to this invention there is provided a method ofelectrophotography comprising the steps of 2 preparing a photosensitiveelement including a highly insulative layer which includes a mixture ofa highly insulative material and a material highly reflective to visiblelight rays, a photoconductive layer manifesting persistent internalpolarization, and an electrode layer transparent to visible light rays,said layers being laminated in the order mentioned and bonded togetherinto an integral structure; applying a first electrode field across thephotosensitive element to deposit a charge of one polarity on thesurface of the highly insulative layer; applying a second electric fieldacross the photosensitive element to deposit a charge of the oppositepolarity on the surface of the highly insulative layer;

' projecting a light image upon the photosensitive layer through theelectrode layer concurrently with the application of the second electricfield whereby to form a latent image corresponding to the light image onthe surface of the highly insulative layer; developing the latent imagewith a colored toner under ambient light to fonn a powder image; andcorrecting the powder image during development by adjusting the densityof toner while observing the powder image against the reflectivebackground provided by the highly insulative layer.

According to another aspect of the invention, a powder image is formedby the same process steps as described hereinabove by using a modifiedphotosensitive element in which a phosphor layer which emits visiblelight rays when excited by radiations such as X-rays or 'y-rays from aradioactive substance, CO for example, is added on the side of thephotoconductive layer opposite the highly insulative layer.

Since the highly insulative layer is highly reflective to visible lightrays from the photoconductive layer, it is easy to observe, correct ormodify the powder image throughout the course of forming the same. Theresulting powder image may be transfer printed to obtain a permanentcopy or can be used as an information source. Thus, for example, in thecase of radiation electrophotography, the powder image may bephotographed to produce a radiograph.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a diagram showing a cross-section of a portion of aphotosensitive element embodying the invention;

FIG. 2 is a cross-section of a modified photosensitive element employinga phosphor layer; and

FIG. 3 is a graph showing the relationship between the percentage ofreflectance and the quantity of TiO: incorporated into thin films ofpolyethylene terephthalate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1 thephotosensitive element for use in electrophotography comprises a firsthighly insulative layer 1 which largely reflects visible light rays, aphotoconductive layer 2 exhibiting persistent internal polarization, atransparent second highly insulative layer 3, a transparent electrodelaayer 4 and a transparent substrate layer 5 which are laminated in theorder mentioned and are bonded together into an integral structure.Different from well known photosensitive elements, the first highlyinsulative layer is made of a highly insulative material of white, milkywhite or similar color and reflects visible light rays well. An example3 of such material involves a mixture of a transparent synthetic resin,such as polyester resin, and a fine pow der of crystals of metalcompounds which reflect visible light rays such as PbO, TiO CaCO andZnS. The

photoconductive layer 2 is "made of a photoconductive material capableof exhibiting temporary persistent internal polarization. The materialmay beeither an inorganic photoconductive material, such as CdS, ZnS,ZnO, PbO, ZnSe, Se, SeTe or an organic photoconductive material such aspolyvinyl carbazol, anthracene and anthraquinone. Many other organic orinorganic photoconductive materials are known in the art. 'As has beendescribed in said US. Pat. Nos. 3,457,070 and 3,536,483 according to themethods of these patents it ispossible to use photoconductive materialshaving low dark resistance and high photosensitivity. 'The secondtransparent highly insulative layer 3 may be made of any organicorinorganic material having sufficient transparency and resistivity. It isessential that the highly insulative layer should not contain pin holes.The transparent electrode layer 4 may be made of a thin layer of SnO ora'vapor deposited film of gold having a thickness of about 100 mg. Thetransparent substrate layer 5 may be made of any transparent materialsuch as glass or a synthetic resin.

A latent image is'forrned in the following manner by using thephotosensitive element described above. Where the photoconductive layer2 comprises a vapor deposited layer of selenium, since the polarity ofthe majority carriers of selenium is positive, in the first step, afirst electric field is applied across the photosensitive element touniformly deposit a positive charge on the surface of thehighlyinsulative layer 1 by means of a removable electrode or 'acoronadischarge unit. Concurrently therewith or after this sensitizing step,uniform light of suitable intensity is projected upon thephotoconductive layer from the side of the transparent highly insulativelayer 3 to stimulate the photoconductive layer with the light. However,such stimulation with light is "not necessary where there are sufficientnumber of charge carriers in the photoconductive layer. In this manner,charge carriers are trapped in the layer of the photoconductive layercontiguous to the interface between the second highly insulative layer 3and the photoconductive layer 2.

In the secondstep'a second electric field is applied acrossthephotosensitive element to deposit a charge of the opposite polarity onthe surface of the first highly insulative layer concurrently theprojection of an information containing light image upon thephotoconductive layer 2 from the rear or bottom side of thephotosensitive element through transparent substrate layer 5,transparent electrode. layer 4 and transparent highly insulative layer3. Atportionsof the photoconductive layer corresponding to dark portionsof the light image the charge carriers are trapped so that they cannotdrift whereas at portio'nscorresponding to bright portions of the lightimage the charge carriers are released "and, since], new electron-holepairs are formed, charge carriers drift; toward the first highlyinsulative layer. As a consequence, a charge pattern or an electrostaticlatent image is formed on the surface of the first highly insulativelayer. Since the latent image is formed on the highly insulative layerit will not be erased by later irradiation of light and can be retainedand developed in a light area. As has been disclosed in thespecification of US. Pat. No. 3,457,070 where the photoconductorcontains deep trap levels as in the case of ZnCdSzAg or CdSzCu it isadvantageous to irradiate the photosensitive element with uniform lightbefore development. The purpose of such post exposure is to depolarizepersistent internal polarization caused by charge carriers trapped atthe deeper trap levels and to extract the surface charge effect alonethus intensifying the apparent net surface charge.

The method of forming a latent image described above is generallyanalogous to those described in US. Pat. Nos. 3,457,070 and 3,536,483.The latent image formed by these prior methods are then developed withcolored toners and transfer printed onto recording papers to obtainso-called hard copies. According to these prior methods, however, it isimpossible to directly observe the latent image formed on thephotosensitive element as an intermediate record or utilize it as asignal source for the following reason.

Where the photoconductive layer is required to have highphotosensitivity and high resolution, the light absorption and hence theresolution of the photoconductive material should also be high. Thesecharacteristics are desirable for electrophotography. However, with sucha material the color of the photoconductive material becomes dark ordeep red by large absorption of visible light rays. When the latentimage formed on the photosensitive element including a transparenthighly insulative layer and a photoconductive layer of suchcharacteristics is developed by means of a colored toner (ordinarilyblack) it is impossible not only to observe the intermediatesteps ofdevelopment but also to utilize the developed powder image as a sourceof information. For example where the evaluation of resolution or theconcentration or contrast of the developed powder image is important asin the case of radiation electrophoto'graphy, whether it is possible ornot to directly observe the progress of the steps ofdevelopmentdeterminesthe commercial value of the electrophotographic orrecording system. These factors also contribute to the commercial valueof the methods of aforementioned patents which enable development of thelatent image under room light. I a The invention is characterized by aphotosensitive element for use in electrophotography capable of forminga latent image which can'be directly observed or corrected during thecourse of forming it or can be used as a type of visible memory. Moreparticularly, as

described above, according to the invention the surface highlyinsulative layer 1 is made of an insulator which largely reflectsvisible light rays so that when the latent image formed on the surfaceof the highly insulative layer is developed by means of charged toner ofblack or dark color, it is possible to directly observe the developedpowder image during the'entire course of development without beingdisturbed by the color of the photoconductive layer. Where the developedpowder image is used as a manuscript 'in'an information transmissionsystem, it is possible to suitably correct the powder image during thedeveloping step to obtain a powder image of the desired quality. Thepowder image can also be utilized as a persistent but erasable visibleimage. Erasure of the powder image can be done by subjecting thephotosensitive element to an AC field and then removing the toner. Suchpowder image can also be transfer printed onto a recording medium, for

instance asheet of recording paper, in a well known manner.

Correction of the tone or deepness of the color of the developed powderimage can be made in the following manner. Where the color is too deepthe powder image is treated with a carrier alone to decrease thedeepness to the desired level. On the other hand, where the color is toopale, the powder image is treated again with the colored toner. Sinceboth dry type and wet type developing systems are not available, theterm powder image herein used is intended to mean a visible imagedeveloped by either one of these two developing systerns.

As shown in FIG. 3, the percentage of reflectance of thin films ofpolyethylene terephthalate varies with the content of a fine powder ofTiO incorporated in the films. A film of polyethylene terephthalatecontaining no TiO was transparent, but to simulate the conditionprevailing in the actual photosensitive element, thin films (each 12p.thick) containing varied amounts of TiO having a grain size of about 0.1p. were placed on a layer of a black SeTe alloy'and the percentage ofsurface reflectance of the respective films was measured with aFujidensitometer sold by Fuji Photo Film Co. Ltd. Japan. The percentageof surface reflectance of a film containing no TiO was 45% and increasedwith the content of TiO At a content of TiO of 5% by weight thepercentage of reflectance was about 80% and the film looked pale whileat a content of TiO of about the percentage of reflectance approached100%. From this and FIG. 3 it can be concluded that the addition of TiOin amounts as low as 1% or 2% provide reflectances which aresignificantly improved over those of elements having no highlyreflective material added to the insulative film. Reflectances fromabout 50 to 100% are useful in carrying out the present invention.Reflectances in the order of 80 to 100% are preferred, however,corresponding to additions of TiO of about 5% by weight to more than20%. Resins containing more than 20% of TiO are difficult to be formedinto thin films, however, and amounts of TiO above 20% do not result inany additional advantage.

To determine the effect of the thickness of the film upon the degree ofreflectance, films having thicknesses of 12p, 24p. and 36p. were testedand it was found that the percentage of reflectance is independent ofthe thickness of the film.

The following specific examples are given by way of illustration, andare not constructed as limiting in any way the scope and spirit of theinvention.

EXAMPLE 1 This example relates to a photosensitive element for use inradiation electrophotography. More particularly, as shown in FIG. 2, aphosphor layer 6 which, when excited by radiations, for example X-raysor 'y-rays emitted by a radioactive substance, emits light of awavelength in the visible range which can excite the photoconductivelayer, is interposed between the electrode layer 4 and the substrate 5of the photosensitive element shown in FIG. 1. Phosphor layer 6 can alsobe interposed between the photoconductive layer 2 and the secondtransparent highly insulative layer 3, or between the second highlyinsulative layer 3 and transparent electrode layer 4. As fully describedin said patents, the purpose of using the second highly insulative layeris to improve the signal-to-noise ratio and use thereof is optional.

A fine powder of [ZnCd] S:Ag:Cl was homogenously dispersed in a cleartransparent synthetic resin binder and the mixture was coated uponsubstrate 5 to form the layer 6 having a thickness of 100 microns. Thefirst highly insulative layer 1 was made of polyester resin containing20% by weight of a fine powder of CaCO The photoconductive layer 2 wasprepared by vacuum depositing a SeTe alloy whereas the secondtransparent highly insulative layer 3 was made of polycarbonate.Substrate 5 was an aluminum sheet having a thickness of 1 mm, which istransparent to radiations.

The photosensitive element was used in the following manner.

In the first step, uniform visible light was projected upon thephotoconductive layer 2 through the first highly insulative layer 1containing CaCO to excite the photoconductive layer. Concurrentlytherewith, positive charge was uniformly deposited on the surface of thehighly insulative layer by means of corona discharge as fully disclosedin U.S. Pat. No. 3,536,483. In the second step, while uniformly chargingnegative charge on the surface of the highly insulative layer 1 withcorona discharge, an X-ray image of an object (not shown) was projectedupon the phosphor layer 6 from the opposite side or through substrate 5.In response to the X-ray image the phosphor layer 6 emitted visible raysto excite the photoconductive layer 2 with -a light image correspondingto the X-ray image,

whereby a latent image was formed on the surface of the first highlyinsulative layer corresponding to the X-ray image. After a suitabletime, typically a few seconds, the latent image was developed with acharged black toner. Since the first highly insulative layer 1 is whiteand does not transmit the light from the photoconductive layer 2, thedeveloped powder image was very clear and it' was possible to observethe detail thereof without the interference caused by the color of thephotoconductive layer. By continuing the developing treatment the degreeof blackness at black portions was increased; It was also possible toobserve the detail of the portions of the powder image of intermediatetones by adjusting the tone of the powder image. Since the developingstep was carried out in the bright or under room light, it was possibleto fully observe the successive steps of development and to makenecessary corrections. Thus, it was possible to obtain a powder image ofthe desired tone or bright-to-black ratio. During the course ofdevelopment, or while the degree of blackness is increasing it waspossible to obtain by observation much valuable information notobtainable with other types of photosensitive elements. Further, it waspossible to transfer print onto a film or paper the powder imagereproduced at an extremely high fidelity to obtain a permanent hardcopy. After transfer printing, the toner remaining on the photosensitiveelement can be removed and the electrostatic latent image can be erasedin a manner well known in the art for preparing the photosensitiveelement for use in the next cycle.

EXAMPLE 2 A latent image was formed and developed with the samephotosensitive element and the same method as in Example 1. The powderimage was used to form a second visible image through an optical systemwhich may be an electrophotographic system or a video system. The powderimage on the surface of the photosensitive element utilized as anintermediate record has an extremely high contrast. In addition, sinceit is possible to use intense light to illuminate the powder image, itis possible to produce a clear radiograph at higher efficiency than byphotographing an X-ray image produced on a fluorescent screen of theconventional 7 X-ray photographic apparatus. Again the photosensitiveelement can be used repeatedly by erasing the powder image and theelectrostatic latent image.

Although the above examples relate to radiation electrophotography, itshould be understood that the advantage of the invention can also beobtained when a' powder image of ordinary printed matter is formed byusing the photosensitive element shown in FIG. 1. In such a case sincethe light image of the object is projected from the under side of thephotosensitive element, all layers 3, 4 and should be transparent tovisible light. Whereas in the case of FIG. 2, layers 3 and 4 should betransparent to visible light, but substrate 5 is required to betransparent to radiations alone but not to visible light. Further, wherephosphor layer 6 is interposed between layers 2 and 3 or between layers3 and 4, those located between the photoconductive layer and thephosphor layer must be transparent to visible light whereas thoselocated on the side of phosphor layer 6 opposite the photoconductivelayer must be transparent to radiations. For this reason in thespecification and claims the term electromagnet wave is used to meaneither the radiations or visible light rays.

Further, the resinous material used to form the first and second highlyinsulative layers 1 and 3 may be any highly insulative material known inthe art. Any inorganic or organic photosensitive photoconductivematerial exhibiting persisting intemal polarization can be used to formthe photoconductive layer 2.

What is claimed is:

l. A method of electrophotography comprising the steps of preparing aphotosensitive element including a highly insulative layer whichincludes a mixture of highly insulative material and a material highlyreflective to visible light rays, said mixture containing at least 5% byweight of said highly reflective material and said highly reflectivematerial comprising fine crystals of metal compound reflecting visiblelight rays, a photoconductive 'layer manifesting persistent internalpolar ization and an electrode layer transparent to visible light rays,said layers being laminated in the order mentioned and bonded togetherinto an integral structure; applying a first electric field across saidphotosensitive element to deposit a charge of one polarity on thesurface of said highly insulative layer; applying a second electricfield across said photosensitive element to deposit a charge of theopposite polarity on the surface of said highly insulative layer;projecting a light image upon said photoconductive layer through saidelectrode layer concurrently with the application of said secondelectric field whereby to form a latent image corresponding to saidlight image on the surface of said highly insulative layer; developingsaid latent image with a colored toner under ambient light to form apowder image; and correcting said powder image during development byadjusting the density of toner while observing the powder image againstthe reflective background provided by said highly insulative layer.

2. The method according to claim 1 wherein said metal compound isselected from the group consisting of PbO, TiO CaCO and ZnS.

3. The method according to claim 1 wherein said mixture contains from 5to by weight of said highly reflective material.

4. The method according to claim 1 wherein said photosensitive elementfurther includes a second highly insulative layer transpaprent tovisible light rays and 8 interposed between said photoconductive layerand said electrode layer.

5. The method according to claim 1 wherein uniform visible light isirradiated upon said photoconductive layer through said highlyinsulative layer concurrently with the application of said firstelectric field.

6. The method according to claim 1 wherein said latent image isirradiated with uniform light before development.

7. A method of radiation electrophotography comprising the steps ofpreparing a photosensitive element including a highly insulative layerwhich includes a mixture of a highly insulative material and a materialhighly reflective to visible light rays, said mixture containing atleast 5% by weight of said highly reflective material and said highlyreflective material comprising fine crystals of metal compoundreflecting visible light rays, a photoconductive layer manifestingpersistent internal polarization, an electrode layer transparent toelectromagnetic waves, and a phosphor layer disposed on the side of saidphotoconductive layer opposite said highly insulative layer, saidphosphor layer emitting visible light rays adapted to excite saidphotoconductive layer when irradiated with radiations, said layers beingbonded together into an integral structure; applying a first electricfield across said photosensitive element to deposit a charge of onepolarity on the surface of said highly insulative layer; applying asecond electric field across said photosensitive element to deposit acharge of the opposite polarity on the surface of said highly insulativelayer; projecting a radiation image upon said phosphor layer from theside thereof opposite said photoconductive layer concurrently with theapplication of said second field so as to cause said phosphor layer toexcite said photoconductive layer with visible light rays whereby toform a latent image corresponding to said radiation image on the surfaceof said highly insulative layer; developing said latent image with acolored toner under ambient light to form a powder image; and modifyingthe tone of said powder image during development by adjusting thedensity of toner while observing the powder image against the reflectivebackground provided by said highly insulative layer.

8. The method according to claim 7 wherein said metal compound isselected from the group consisting of PbO, TiO CaCO and ZnS.

9. The method according to claim 7 wherein said mixture contains from 5to 20% by weight of said highly reflective material.

10. The method according to claim 7 wherein said photosensitive elementfurther includes a second highly insulative layer transparent to visiblelight rays and interposed between said photoconductive layer and saidphosphor layer.

11. The method according to claim 7 wherein uniform visible light isirradiated upon said photoconductive layer through said highlyinsulative layer concurrently with the application of said firstelectric field.

12. The method according to claiam 7 wherein said latent image isirradiated with uniform light before development.

13. A method according to claim 1 wherein the percentage lightreflectance of said insulative layer in said photosensitive element isat least 50%.

14. A method according to claim 13 wherein the percentage lightreflectance of said insulative layer in said photosensitive element isat least 3,928,031 9 l A method according to claim 7 wherein the penpercentage light reflectance of said insulative layer in saidphotosensitive element is at least 80%. centage light reflectance ofsaid msulatlve layer In said photosensitive element is at least 50%.

16. A method according to claim 15 wherein the UNITED STATES PATENT ANDTRADEMARK OFFICE (IETHTCATE 0F CORRECTION Q PATENT NO. 3,928,031

DATED December 23, 1975 INVENTOR(S) Koichi Kinoshita it is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Heading of the Patent, insert the following:

-Foreign Application Priority Data Q August 10, 1970 Japan 69879/1970Signed and Sealed this eighteenth Day of Ma /I976 [SEAL] Arrest:

}. RUTH c. msoru c. MARSHALL DANN Arresting Officer (ommr'ssimrr'rnflarenrs aml Trade-marks I 5 I Q t l,. i w i li

1. A METHOD OF ELECTROPHOTOGRAPHY COMPRISING THE STEPS OF PREPARING APHOTOSENSITIVE ELEMENT INCLUDING A HIGHLY INSULATIVE LAYER WHICHINCLUDES A MIXTURE OF HIGHLY INSULATIVE MATERIAL AND A MATERIAL HIGHLYREFLECTIVE TO VISIBLE LIGHT RAYS, SAID MIXTURE CONTAINING AT LEAST 5% BYWEIGHT OF SAID HIGHLY REFLECTIVE MATERIAL AND SAID HIGHLY REFLECTIVEMATERIAL COMPRISING FINE CRYSTALS OF METAL COMPOUND REFLECTING VISIBLELIGHT RAYS, A PHOTOCONDUCTIVE LAYER MANIFESTING PERSISTENT INTERNALPOLARIZATION AND AN ELECTRODE LAYER TRANSPARENT TO VISIBLE LIGHT RAYS,SAID LAYERS BEING LAMINATED IN THE ORDER MENTIONED AND BONDED TOGETHERINTO AN INTEGRAL STRUCTURE; APPLYING A FIRST ELECTRIC FIELD ACROSS SAIDPHOTOSENSITIVE ELEMENT TO DEPOSIT A CHARGE OF ONE POLARITY ON THESURFACE OF SAID HIGHLY INSULATIVE LAYER; APPLYING A SECOND ELECTRICFIELD ACROSS SAID PHOTOSENSITIVE ELEMENT TO DEPOSIT A CHARGE OF THEOPPOSITE POLARITY ON THE SURFACE OF SAID HIGHLY INSULATIVE LAYER;PROJECTING A LIGHT IMAGE UPON SAID PHOTOCONDUCTIVE LAYER THROUGH SAIDELECTRODE LAYER CONCURRENTLY WITH THE APPLICATION OF SAID SECONDELECTRIC FIELD WHEREBY TO FORM A LATENT IMAGE CORRESPONDING TO SAIDLIGHT IMAGE ON THE SURFACE OF SAID HIGHLY INSULATIVE LAYER; DEVELOPINGSAID LATENT IMAGE WITH A COLORED TONER UNDER AMBIENT LIGHT TO FORM APOWDER IMAGE; AND CORRECTING SAID POWDER IMAGE DURING DEVELOPMENT BYADJUSTING THE DENSITY OF TONER WHILE OBSERVING THE POWDER IMAGE AGAINSTTHE REFLECTIVE BACKGROUND PROVIDED BY SAID HIGHLY INSULATIVE LAYER. 2.The method according to claim 1 wherein said metal compound is selectedfrom the group consisting of PbO, TiO2, CaCO3 and ZnS.
 3. The methodaccording to claim 1 wherein said mixture contains from 5 to 20% byweight of said highly reflective material.
 4. The method according toclaim 1 wherein said photosensitive element further includes a secondhighly insulative layer transpaprent to visible light rays andinterposed between said photoconductive layer and said electrode layer.5. The method according to claim 1 wherein uniform visible light isirradiated upon said photoconductive layer through said highlyinsulative layer concurrently with the application of said firstelectric field.
 6. The method according to claim 1 wherein said latentimage is irradiated with uniform light before development.
 7. A methodof radiation electrophotography comprising the steps of preparing aphotosensitive element including a highly insulative layer whichincludes a mixture of a highly insulative material and a material highlyreflective to visible light rays, said mixture containing at least 5% byweight of said highly reflective material and said highly reflectivematerial comprising fine crystals of metal compound reflecting visiblelight rays, a photoconductive layer manifesting persistent internalpolarization, an electrode layer transparent to electromagnetic waves,and a phosphor layer disposed on the side of said photoconductive layeropposite said highly insulative layer, said phosphor layer emittingvisible light rays adapted to excite said photoconductive layer whenirradiated with radiations, said layers being bonded together into anintegral structure; applying a first electric field across saidphotosensitive element to deposit a charge of one polarity on thesurface of said highly insulative layer; applying a second electricfield across said photosensitive element to deposit a charge of theopposite polarity on the surface of said highly insulative layer;projecting a radiation image upon said phosphor layer from the sidethereof opposite said photoconductive layer concurrently with theapplication of said second field so as to cause said phosphor layer toexcitE said photoconductive layer with visible light rays whereby toform a latent image corresponding to said radiation image on the surfaceof said highly insulative layer; developing said latent image with acolored toner under ambient light to form a powder image; and modifyingthe tone of said powder image during development by adjusting thedensity of toner while observing the powder image against the reflectivebackground provided by said highly insulative layer.
 8. The methodaccording to claim 7 wherein said metal compound is selected from thegroup consisting of PbO, TiO2, CaCO3 and ZnS.
 9. The method according toclaim 7 wherein said mixture contains from 5 to 20% by weight of saidhighly reflective material.
 10. The method according to claim 7 whereinsaid photosensitive element further includes a second highly insulativelayer transparent to visible light rays and interposed between saidphotoconductive layer and said phosphor layer.
 11. The method accordingto claim 7 wherein uniform visible light is irradiated upon saidphotoconductive layer through said highly insulative layer concurrentlywith the application of said first electric field.
 12. The methodaccording to claiam 7 wherein said latent image is irradiated withuniform light before development.
 13. A method according to claim 1wherein the percentage light reflectance of said insulative layer insaid photosensitive element is at least 50%.
 14. A method according toclaim 13 wherein the percentage light reflectance of said insulativelayer in said photosensitive element is at least 80%.
 15. A methodaccording to claim 7 wherein the percentage light reflectance of saidinsulative layer in said photosensitive element is at least 50%.
 16. Amethod according to claim 15 wherein the percentage light reflectance ofsaid insulative layer in said photosensitive element is at least 80%.